1. Field of the Invention
The proposed invention relates to radio engineering, particularly, to the methods of multipath signal reception using an adaptive antenna array in CDMA communication systems and can be applied to receivers of cellular communications systems.
2. Description of the Related Art
During the last decade, CDMA systems have been intensively deployed worldwide. The necessity of increasing the capacity of these systems forced the use of adaptive antenna arrays at base stations, which is reflected in 3G communications standards.
An adaptive antenna array carries out a weighed combining of the signals from antenna array elements and enables space processing of signals. The processing parameters (weight factors of antenna array elements determining an antenna pattern) are automatically varying so that to provide the best (or acceptable) desired signal reception conditions in a variable environment that interferes with the signal. The use of an adaptive antenna array in CDMA cellular communications systems enables considerable improvement of a number of system parameters, particularly, system capacity, communication quality, coverage area, and mobile user power consumption.
During generation of the weight factor vector of antenna array elements, according to the selected optimality criterion, it is possible to obtain space filtration effects, mitigate powerful interference and perform angle spreading in a multipath channel.
A device of adaptive antenna array beam control operating according to the algorithm of direct inversion of the co-variance matrix of the input signal is disclosed in Russian patent application #98102722/09. The device implements the method of adaptive antenna array beam control according to the criterion of the minimum mean-squared deviation of the output signal of an antenna array from the reference signal. In this application, in order to find the vector of weight factors, the co-variance matrix of the input signal is generated and inversed at the antenna array elements.
The disadvantage of this algorithm is relatively high implementation complexity, and considerable reduction of the accuracy of determination of the weight vector in case the co-variance matrix is badly conditioned.
A method of adaptive antenna array beamforming is described in EP patent EP 0 899 894 A2. “Smart Antenna Receiver and Signal Receiving Method,” by Park Jin-Soo. The described adaptive algorithm forms a beam according to the criterion of minimum instant squared deviation of the output antenna array signal from the reference signal. The weight vector is determined numerically by the method of fastest descent.
In order to obtain the weight vector close to the optimum, a great number of iterations is needed. The algorithm is disadvantageous when reception conditions are varying rapidly.
A method of adaptive antenna array beamforming is disclosed in U.S. Pat. No. 6,108,565 to Shimon B. Scherzer, “Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement”, issued Aug. 22, 2000. The described method, is as follows.
The weight factors of path elements are formed, for which the following operations are carried out over each path:                the input signal is demodulated at antenna array elements,        the demodulated input signal is fast Hadamar transformed at antenna array elements forming the input signal matrix,        the input signal matrix is multiplied by the reference signal matrix,        an estimate of the angle of arrival of the input path signal is found by analyzing the result of multiplication of the input signal matrix and the reference signal matrix,        the current weight vector is defined as the vector corresponding to the estimate of angle of arrival of the input path signal.        
The current values of the weight factors of paths are used for the output ones and determine an adaptive antenna array beam.
The reference signal matrix is determined by the signals corresponding to the pre-determined discrete hypotheses of angle of arrival of the input signal. The estimate of angle of arrival of the input signal θ determines the weight vector according to the equationw=[1, e−jφ, e−j2φ, . . . , e−j(N−1)φ],  (1)where
      ϕ    =                            2          ⁢          π                λ            ⁢      d      ⁢                          ⁢      sin      ⁢                          ⁢      θ        ,λ—wavelength, d—spacing/distance between antenna array elements and N is the number of processing channels in a path signal processing unit.
FIG. 1 shows a device for implementing this method. The device comprises L path signal processing units, one shown in FIG. 1. Each of L path signal processing units includes N parallel channels composed of successively linked complex multipliers 2.1–2.N and FHT (Fast Hadamar transform) units 3.1–3.N, and further comprises reference signal generator 1, matrix multiplier and multiplication result analyzer 4, and weight vector former 5. The first inputs 1–N of complex multipliers 2.1–2.N are the signal inputs of the device; the second inputs are reference signals output by reference signal generator 1. The output of each FHT unit 3.1–3.N is linked to the corresponding input of matrix multiplier and multiplication result analyzer 4, the output of which is input to the input of weight factor former 5. The output of weight factor former 5 is the current vector of path weight factors and the output of the device.
The device operates in the following manner. In each of L signal processing units, the complex input signal is supplied to the first (signal) inputs 1–N of complex multipliers 2.1–2.N. A reference PN sequence is applied to the second (reference) inputs of complex multipliers 2.1–2.N from the output of reference signal generator 1. The state of reference signal generator 1 corresponds to the path signal time position in the received multipath signal. Complex demodulated signals are outputfrom complex multipliers 2.1–2.N and inputto corresponding FHT units 3.1–3.N, where the input signal is decomposed in the Hadamar function basis. Input signal spectrums are output from the FHT units 3-1–3-N and supplied to N inputs of matrix multiplication and multiplication result analyzer 4. In unit 4 the input signal matrix is multiplied by the reference signal matrix. The input signal matrix is formed from the input signal spectrums. The reference signal matrix is determined by the signals corresponding to the pre-determined discrete hypotheses on angle of arrival of the input signal. In addition, matrix multiplier and multiplication result analyzer 4 analyzes the result of multiplication of the input signal matrix by the reference signal matrix and the estimate of angle of arrival of the input path signal is acquired. The estimate of angle of arrival of the input path signal is output by matrix multiplier and multiplication result analyzer 4 and input to the weight vector former 5. Weight vector former 5 based on the estimate of angle of arrival of the input path signal forms at its output the current vector of weight factors of the path that is the output signal of the device.
One disadvantage of this system is the impossibility of canceling interferences caused by signals whose angle of arrival differs inconsiderably from the desired signal arrival angle.